Packaging of semiconductor dies using a lead frame is well known in the art. Referring to FIG. 1, there is shown a cross-sectional view of a packaged semiconductor die using a lead frame. A plan view of a lead frame 20 of the prior art is shown in FIG. 20. The packaged die 10 comprises a lead frame 20 having a central recessed portion 14 and a plurality of spaced apart leads 12. An integrated circuit die 22 has a top surface 24 and a bottom surface 26, with the top surface 24 having a plurality of bonding pads which electrically connect to the various circuit elements in the integrated circuit die 24. The die 22 is positioned in the recessed portion 14, such that the bottom surface 26 of the die 22 rests on the recessed portion 14, through a conductive paste and is electrically connected to the recessed portion 14. The recessed portion 14 is formed of a conductive material. A plurality of bonding wires 30 connect certain of the bonding pads on the top surface of the die 22 to the top side of certain of the leads 12. Each of the leads 12 has a bottom side 32, opposite to the top side, which are will be connected to various electrical pads on a printed circuit board (PCB) for system application. The recessed portion 14 is also connected to an electrical contact, typically ground, on the printed circuit board on the same side as the bottom side 32 of the leads 12.
In the method of the prior art, the die 22 is first placed on the recessed portion 14 of a preformed lead frame 20. The die 22 may be attached to the recessed portion 14 by adhesive to prevent movement of the die 22 in the subsequent steps. A wire bonding machine bonds certain of the bonding pads of the die 22 to the top side of certain of the leads 12. Once all the wires have been so bonded, resin is injected into a mold chest to encapsulate and insulate the die 22, the wires 30 and the top sides of the leads 12. The structure is then singulated or cut and each packaged die can then be used to connect to other packaged semiconductor devices, by well known techniques, such as by soldering the packaged die 10 on a printed circuit board PCB. In that event, the bottom sides 32 of the leads 12 and the bottom surface of the recessed portion 14, can be soldered to the PCB. Thus, the packaged semiconductor die of the prior art, as shown in FIG. 1 has electrical connections to only one side of the packaged die for connection to a printed circuit board.
Referring to FIG. 2a, there is shown the first step in another method of forming a packaged semiconductor die 40 of another prior art, which is very similar to the method shown and described in FIG. 1. In this method, the method begins with a slab of copper alloy 42, having a top side and a bottom side. Photoresist 44 is applied to both the top side and the bottom side, and a masking step is formed on both sides. After the unmasked portions of the photoresist 44 are removed, a solderable material 46, such as tin is sputtered to fill the removed portions. The resultant structure is shown in FIG. 2b. 
The photoresist 44 is then removed, leaving the solderable material 46 on the copper alloy 42. The resultant structure is shown in FIG. 2c. With the solderable material 46 as a mask, a wet etch of the copper is performed on the top side of the copper alloy 42. The etch forms a recessed central portion 50. The resultant structure is shown in FIG. 2d. 
A die 22 is placed in the recessed cavity 50 with the bonding pads of the die 22 facing outwardly. Wires are then bonded to the bonding pads of the die 22 and to the solderable posts 46 on the top side. An insulator encapsulation material is then applied to the top side of the die 22 and the etched copper alloy at the bottom side. The resultant structure is then singulated or cut and the result is shown in FIG. 2f. 
One of the differences between the method shown in FIG. 1 and the method shown in FIGS. 2(a-f) is that in the method of FIG. 1, a preformed lead frame 20 is used. In contrast, in the method shown in FIGS. 2(a-f) a slab of copper alloy is etched to form a lead frame. In both methods, however, wire bonding electrically connects the bonding pads of the die 22 to the top sides of the posts of the lead frame. As a result, the resultant packaged structure is incapable of being electrically connected together in a POP (Packaged-on-Package) configuration, or where on packaged die is electrically connected in a stacked manner to another packaged die.
Finally, panel based packaging of dies with patterned conductors by sputtering or plating serving as electrical connections to bonding pads of semiconductors dies are also well known in the art. See for example U.S. Pat. Nos. 7,224,061; 7,514,767; and 7,557,437.